Organic electroluminescent (EL) devices or organic light-emitting devices (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED comprises, in sequence, an anode, an organic EL medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al. demonstrated highly efficient OLEDs using such a layer structure in “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987) and in commonly assigned U.S. Pat. No. 4,769,292. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly includes of a host material doped with a guest material. Further, there are other multilayer OLEDs that contain additional functional layers, such as a hole-injecting layer (HIL), and/or an electron-injecting layer (EIL), and/or an electron-blocking layer (EBL), and/or a hole-blocking layer (HBL) in the devices. At the same time, many different types of EL materials are also synthesized and used in OLEDs. These new structures and new materials have further resulted in improved device performance.
In an OLED, crystallization of an organic layer is detrimental to the device performance, especially if the ETL is the one undergoing the crystallization process in the device. During device operation, if the temperature inside of a device (defined as device temperature) is higher than a glass transition temperature (Tg) of an organic layer in the OLED, the organic layer will change its film formation from an amorphous state to a polycrystalline formation. This change will not only cause a film morphology change, but also cause a possible change in its ionization potential (Ip) and/or its electron energy band gap (Eg). As a result, electrical shorts can occur, carrier injection can deteriorate, or luminance efficiency can be reduced. Therefore, selecting high Tg materials, especially high Tg electron-transporting materials, is very necessary for the application of OLEDs. Tg of organic materials can be obtained using a technique such as differential scanning colorimetry.
Tris(8-hydroxyquinoline)aluminum (Alq), one of the metal chelated oxinoid compounds, has been a commonly used electron-transporting material in OLEDs since Tang et al. disclosed its use in “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987). Alq has a reasonably high Tg (about 172° C.). This property facilitates the operational stability of the OLEDs at a device temperature up to its Tg. However, the electron mobility of Alq is not quite as good as is expected. In order to improve the electron-transporting property in OLEDs, efforts are being made to try to use some other electron-transporting materials, such as other metal chelated oxinoid compounds, butadiene derivatives, heterocyclic optical brighteners, benzazoles, oxadiazoles, triazoles, pyridinethiadiazoles, triazines, and some silole derivatives. Among those materials, it is found that 4,7-diphenyl-1,10-phenanthroline (Bphen) has a very high electron mobility.
Due to its high electron mobility and suitable energy band structure, Bphen as an electron-transporting material in an ETL of an OLED can efficiently transport electrons from the cathode into the LEL resulting in high luminous efficiency and low drive voltage. Unfortunately, Bphen has a low Tg (about 60° C.), and a vacuum deposited amorphous Bphen layer in an OLED can be readily changed into a polycrystalline layer during operation, which results in a sudden drop in luminance and a sudden increase in drive voltage. Its operational lifetime is no longer than 20 hrs if the device is operated at 70° C., substantially minimizing the effectiveness of this material in an OLED.